Complex as-grown nanodomain patterns are studied with piezoresponse force microscopy in Sr_xBa_(1-x)Nb_2O_6 single crystals at variant Sr/Ba molar ratios, 0.4 ≤ x ≤ 0.75. They reflect random-field Ising model (RFIM) ferroelectricity, which crosses over into relaxor behavior at increasing x. This is explained by an increase of the polar disorder within the open tungsten-bronze crystal structure giving rise to enhanced quenched random electric fields at simultaneous decrease of the ferroelectric ordering temperature. The strongly anisotropic domain shapes, their fractal properties and size distributions, and their metastable occurrence above the nominal Curie temperature are in accordance with predictions for the RFIM.
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